Hot-dip galvannealed steel sheets (hereinafter sometimes abbreviated as “GA steel sheets”) are obtained by heating hot dip galvanized steel sheets (GI steel sheets) to diffuse Fe in basis steel sheets into plated layers so that Fe and Zn are alloyed. Since GA steel sheets have excellent strength, weldability, corrosion resistance after being painted and other properties, it is used, for example, for a structural member (a member serving as an energy absorber during collision) of automobiles.
Such a GA steel sheet sometimes has the problem that the plated layer peels off in the form of powders during forming, which is called powdering. In recent years, improvement in tensile strength is required for steel sheet for automobiles for the purpose of improving fuel efficiency by reduction of body weight improving collision safety. Since this improvement intensile strength makes forming conditions severe during pressing, damage caused in the plated layer is further increased, causing powdering more easily.
An example of widely known methods for improving powdering resistance of a GA steel sheet is reducing the iron concentration in the Fe—Zn alloy plated layer to reduce the brittle Γ phase. In addition, for example, Japanese Patent No. 2695259 discloses powdering resistance and flaking resistance can be improved by adjusting the amounts of the ζ phase, δ1 phase and Γ phase in the plated layer and inhibiting formation of the Γ phase at the interface of the basis iron (basis steel sheet) to further limit the surface roughness to a low level. However, these means can only produce insufficient effect in improving powdering resistance on the plated layers in recent steel sheets having high tensile strength.
Japanese Published Unexamined Patent Application No. 2002-302753 discloses a hot dip galvannealed steel sheet excellent in press formability (sliding property during press molding) and chemical treatability having a flat portion in which an oxide layer having a thickness of 10 nm (100 Å) or more is formed on the surface of the plated layer and a Zn/Al ratio (atomic %) in the surface layer of the flat portion is 2.0 to 8.0. However, this cited invention only aims to improve the press formability and chemical treatability of the GA steel sheet, and does not consider powdering resistance.
Furthermore, in this cited invention, the thick “oxide layer” which has the function to improve press formability means “a layer comprising one or more oxides and/or hydroxides of Zn, Fe, Al and other metal elements”. In contrast, the “Zn/Al ratio on the surface layer” in this cited invention is used as an index for unevenness on the surface layer of the oxide layer for imparting both press formability and chemical treatability. In the cited invention, this “Zn/Al ratio” is merely a value of the surface layer in the flat portion of the plated layer, and it is not thought that the entire “oxide layer”, that is, even the deepest part of the oxide layer, has this ratio. In other word, although the cited invention considers the thickness of the “oxide layer”, the thickness of the region having the specific “Zn/Al ratio” is not considered at all.
Incidentally, steel sheets for automobiles are often press-formed into complicated shapes. Therefore, GA steel sheets are further required to have excellent formability (elongation). However, increased strength in a steel sheet deteriorates formability, a steel sheet having both strength and formability (improvement in the balance of strength and ductility) is required.
For these reasons, TRIP steel sheets are attracting attention as basis steel sheets used for GA steel sheets: this TRIP steel sheet is manufactured by producing retained austenite (hereinafter sometimes referred to as “retained γ”) in its structure and allowing this retained γ to undergo induced transformation (transformation induced plasticity “TRIP”) during deformation in working, thereby producing excellent ductility. Typical examples of base phases of the TRIP steel sheets include polygonal ferrite and bainitic ferrite, as well as tempered martensite, tempered bainite and the like. In TRIP steel sheets, a base phase structure is introduced by adjusting a cooling rate after hot rolling or by other means; the steel sheet at a ferrite-austenite two-phase region temperature or austenite single-phase region temperature is cooled according to a specific pattern; and is then heated to and held at a predetermined temperature (austempering), whereby the retained γ is introduced.
Japanese Published Unexamined Patent Application No. 2002-235160 discloses a TRIP steel sheet which comprises polygonal ferrite and bainitic ferrite as the base phase structure. This document mainly discusses a Gl steel sheet, and describes that the concentration of C (Cγ) in the retained γ greatly affects the characteristics of the TRIP steel sheet and the higher the amount of Cγ contained (for example, Cγ≧0.8%) the better the ductility such as elongation. However, this document does not specifically describe GA steel sheets.
Japanese Published Unexamined Patent Application No. 2005-146301 discloses a TRIP steel sheet comprising tempered martensite and ferrite as the base phase structure, and both Gl steel sheets and GA steel sheets are shown as examples. This document describes that a preferred alloying temperature for the GA steel sheets is 450 to 600° C., but it does not refer to the concentration of C (Cγ) in the retained γ.
TRIP steel sheet utilizes an excellent ductility improving function by the retained γ. However, there is the disadvantage that the retained γ produced by austempering is transformed into cementite and ferrite if its alloying is not property performed and the amount of the retained γ in the GA steel sheet is reduced. That is, although excellent balance of strength and ductility is initially obtained due to production of the retained γ in the Gl steel sheet, part of the retained γ disappears in the Gl steel sheet in the process of alloying the Gl steel sheet. Therefore, the GA steel sheet has the problem that a desired balance of strength and ductility is not effectively exhibited in some cases.
A technique for increasing the formability of a high-strength hot dip galvanized steel sheet is disclosed in Japanese Examined Patent Publication No. S62-40405, which discusses converting the metal structure of the steel sheet into a dual-phase (“DP”) containing a low temperature transformation phase mainly consisting of a ferrite basis and martensite. However, the strength of the DP steel sheet disclosed in this document is about 600 MPa, but even higher strength is required.
Japanese Published Unexamined Patent Application No. H9-13147 also describes a high tensile strength hot dip galvannealed steel sheet with increased moldability and a strength of 800 MPa or more. This document describes that Si is added in an amount of 0.4% or more to enhance the strength of the steel sheet and also impart a dual phase structure of ferrite and martensite to the metal structure of the steel sheet. However, this document does not pay attention to the relationship between Si and the balance of strength and ductility, and the balance of strength and ductility is deteriorated in some cases.
The present invention was accomplished in such situations, and its primary object is to provide a high-strength hot dip galvannealed steel sheet having high powdering resistance (particularly a high tensile strength steel sheet). It is another object of the present invention to provide a high-strength hot dip galvannealed steel sheet which has excellent powdering resistance and exhibits excellent balance of strength and ductility, and a useful method for producing such a hot dip galvannealed steel sheet.